The present invention relates to a sound processing apparatus and method with fading effect, particularly to one which only needs to separate a pulse code modulation signal into two or more sections and then proceeds conversions including at least a DC bias conversion and a fading-out conversion, thereafter combining the two converted signals to achieve a fading effect to a sound and retaining the DC level of the sound so as to eliminate distortion caused by unwanted saturation or cut-off of transistors.
Integrated circuits used in the field of synthesization of sound have been very popular for years. For generating different sound effects, different kinds of devices such as sound generators, beat generators, dual-sound generators have been provided. A conventional method utilizes a fade-in or fade-out step, i.e., to gradually increase or decrease the sound volume, for achieving sound source moving effect, i.e., for achieving stereoscopic or surround sound effects. For gradually varying the sound volume in the field of digital sound synthesization, floating point operation is applied on the sound signals.
Hereunder are formulas for achieving fade-out effect. FIG. 2 provides an explanation of these formulas. Referring to FIG. 2, PCMth represents a transition threshold of a PCM signal, i.e., the DC level of the PCM signal.
If PCM&gt;PCMth (for example PCM1) EQU PCM (fading)=PCMth+(PCM-PCMth) * r (1)
If PCM&lt;PCMth (for example PCM2) EQU PCM (fading)=PCMth-(PCMth-PCM) * r (1)
where PCM represents any point of the PCM signal of FIG. 2; and
where r is a fading factor which is in the range between 0 and 1, i.e. 0&lt;r.ltoreq.1
Since the above two formulas apply multiplication of floating point (please note that the fading factor r is ranged between 0 and 1), a mathematics numeric operation unit is required thus resulting in a complicated structure and high cost.
Conventionally, the mathematics numeric operation unit is not used but is replaced with a digital/analog converter whose output signal is decreased or increased internally. Although this way can reduce cost, it suffers from distortion problems such as unwanted saturation or cut-off of transistors. For example, a transistor 41 as shown in FIG. 4 is apt to be rendered in a saturation status or cut-off status if the DC component of the input signal I/P is increased or decreased over a threshold value. More particularly, if the signal level of the input signal I/P is increased, the DC component thereof is apt to exceed a first threshold level (see MAX of FIG. 2) thus rendering the transistor 41 into a saturation status and resulting in a saturation distortion. If the signal level of the input signal I/P is decreased, the DC component thereof is apt to be less than a second threshold level (see min of FIG. 2) thus rendering the transistor 41 into a cut-off status and resulting in a cut-off distortion.
Based on the above consideration, it is requisite to provide a new device for achieving fading effect without using a floating point operation device or resulting in distortion due to unwanted saturation or cut-off of transistors.